Zinc (Zn)-plated steel sheets have been frequently used as automotive steel sheets for the purpose of surface appearance and corrosion resistance. Zn-plated steel sheets are broadly classified as hot-dip galvanized (GI) steel sheets and electro galvanized (EG) steel sheets. Hot-dip galvanized steel sheets are manufactured by using a method of plating a steel sheet through dipping in a hot-dip galvanizing bath, and electro galvanized steel sheets are plated by using an electroplating method.
Hot-dip galvanized steel sheets have better price competitiveness than electro galvanized steel sheets, but demand therefor is limited due to poor image clarity and appearance quality after coating. In order to address the foregoing limitations, a technique of refining spangles in a plated layer by using a technique (Patent Document 1) of electrostatic atomization of a phosphate solution on the zinc-plated layer after galvanizing has been developed. When the technique is used, a hot-dip galvanized steel sheet (GI-ACE steel sheet) having a spangle diameter ranging from about 50 μm to about 1000 μm in a zinc plated layer may be manufactured. The GI-ACE steel sheet obtained through the refinement of the spangles in the plated layer has excellent coating surface characteristics, such as coatability, corrosion resistance, and surface appearance, and has superior economic factors due to an insignificant change in price in comparison to that of a general GI steel sheet. That is, the GI-ACE steel sheet may be more suitable for the requirements of automotive steel sheets in comparison to a typical GI steel sheet.
Meanwhile, when automobiles are manufactured, structural adhesives may be, used in order to reinforce collision performance or as a substitute for welding. In order to use a structural adhesive, excellent adhesiveness is required to be secured through an impact peel test at an ultra-low temperature (about −40° C.).
In the case that the impact peel test is performed on the typical GI steel sheet, fractures may occur in the adhesive. This denotes that the zinc-plated layer has excellent adhesiveness. In contrast, with respect to the GI-ACE steel sheet, fractures may occur in the zinc-plated layer.
In general, it is known that brittle fractures occur in zinc in a (0001) plane and three {10-10} planes at low temperatures, and activity of zinc fracture mechanisms changes according to temperature from a brittle+grain boundary+ductile fracture type at a high temperature to a brittle fracture type at a low temperature. It is also known that fractures in a plated layer occur at spangle boundaries of the plated layer or an interface between a base steel sheet and the plated layer.
The occurrence of fractures at spangle boundaries or an interface may be due to the fact that voids are generated in spangle boundaries after hot-dip galvanizing, because volume changes due to shrinkage occurring when zinc solidifies. Also, since the difference between a thermal expansion coefficient of zinc (about 1.5 to 6.1×10−5/K) and a thermal expansion coefficient of iron (about 1.18×10−5/K) exists, the fracture at an interface may occur due to thermal misfit. Further, it is known that a relatively large amount of stress is applied to the spangle boundaries having a preferred orientation of the plated layer integrated on a (0001) plane and, as a result, fractures start at an interface between the base steel sheet and the plated layer.
Therefore, with respect to the GI-ACE steel sheet, the interface between the plated layer and the base steel sheet or the spangle boundaries having a preferred orientation integrated on a (0001) plane may act as crack initiation sites to generate a fracture.
In order to address the foregoing limitations of the GI-ACE steel sheet, a technique (Patent Document 2) of increasing an average diameter of spangles in the plated layer has been developed. However, surface qualities, such as image clarity, galling resistance, and corrosion resistance, may deteriorate as the diameter of spangles increases.
(Patent Document 1) Korean Patent Application Laid-Open Publication No. 2006-0076214
(Patent Document 2) Korean Patent Application Laid-Open Publication No. 2011-0075612